Cleanup old work item type id backfill on issues table
What does this MR do and why?
In !71869 (merged) we scheduled a long running migration to backfill issues.work_item_type_id
so we can validate a not null constraint. We got a setback with the constraint, so it had to be reverted. After some fixes, this migration will need to be rescheduled, but for the time being it's getting in the way of the code to schedule background migrations (!95133 (merged)).
We can later recover some of the code that is deleted here, when the migration is scheduled again.
Deleting existing records from the DB as we are removing the batching strategy that some environments might already been using.
MR acceptance checklist
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Related to #338004 (closed)